Switched mode power converter controller with ramp time modulation

ABSTRACT

A controller to regulate a power converter includes a terminal coupled to receive an enable signal including enable events representative of an output of the power converter. A drive circuit is coupled to generate a drive signal to control switching of a power switch to control a transfer of energy from an input of the power converter to the output of the power converter. The drive circuit is coupled to turn on the power switch when an enable event is received in the enable signal. A current limit threshold generator is coupled to the drive circuit to generate a current limit threshold signal. The current limit threshold signal varies each switching cycle of the power switch. A time between consecutive enable events in the enable signal is the switching cycle of the power switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/795,785, filed on Oct. 27, 2017, now pending, which is a continuationof U.S. patent application Ser. No. 14/961,575, filed on Dec. 7, 2015,now U.S. Pat. No. 9,837,911, which is a continuation of U.S. patentapplication Ser. No. 13/800,769, filed on Mar. 13, 2013, now U.S. Pat.No. 9,246,392. U.S. patent application Ser. No. 15/795,785, U.S. Pat.No. 9,837,911, and U.S. Pat. No. 9,246,392 are hereby incorporated byreference.

BACKGROUND INFORMATION Field of the Disclosure

The present invention relates generally to power converters, and morespecifically to controllers for switched mode power converters.

Background

Electronic devices use power to operate. Switched mode power convertersare commonly used due to their high efficiency, small size and lowweight to power many of today's electronics. Conventional wall socketsprovide a high voltage alternating current. In a switching powerconverter a high voltage alternating current (ac) input is converted toprovide a well regulated direct current (dc) output through an energytransfer element. In operation, a switch is utilized to provide thedesired output by varying the duty cycle (typically the ratio of the ONtime of the switch to the total switching period), varying the switchingfrequency or varying the number of pulses per unit time of the switch ina switched mode power converter.

The switched mode power converter also includes a controller. Outputregulation may be achieved by sensing and controlling the output in aclosed loop. The controller may receive a signal representative of theoutput and the controller varies one or more parameters in response tothe signal to regulate the output to a desired quantity. Various modesof control may be utilized such as pulse width modulation (PWM) controlor ON/OFF control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a diagram illustrating an example switched mode powerconverter utilizing a controller, in accordance with the teachings ofthe present invention.

FIG. 2A is a graph illustrating an example current limit thresholdwaveforms, in accordance with the teachings of the present invention.

FIG. 2B is a timing diagram illustrating an example current limitthreshold and enable signal, in accordance with the teachings of thepresent invention.

FIG. 3 is a diagram illustrating one example of the controller of FIG.1, in accordance with the teachings of the present invention.

FIG. 4 is a timing diagram illustrating various example waveformsrepresenting signals of the example controller of FIG. 3, in accordancewith the teachings of the present invention.

FIG. 5 is a timing diagram illustrating in increased detail variousexample waveforms representing signals shown in FIG. 4 in accordancewith the teachings of the present invention.

FIG. 6 is another timing diagram illustrating various example waveformsrepresenting signals of the example controller of FIG. 3 in accordancewith the teachings of the present invention.

FIG. 7 is a diagram illustrating another example of the controller ofFIG. 1, in accordance with the teachings of the present invention.

FIG. 8 is a timing diagram illustrating various example waveformsrepresenting signals of the example controller of FIG. 7, in accordancewith the teachings of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. In other instances, well-known materials or methods have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment”, “anembodiment”, “one example” or “an example” means that a particularfeature, structure or characteristic described in connection with theembodiment or example is included in at least one embodiment of thepresent invention. Thus, appearances of the phrases “in one embodiment”,“in an embodiment”, “one example” or “an example” in various placesthroughout this specification are not necessarily all referring to thesame embodiment or example. Furthermore, the particular features,structures or characteristics may be combined in any suitablecombinations and/or subcombinations in one or more embodiments orexamples. Particular features, structures or characteristics may beincluded in an integrated circuit, an electronic circuit, acombinational logic circuit, or other suitable components that providethe described functionality. In addition, it is appreciated that thefigures provided herewith are for explanation purposes to personsordinarily skilled in the art and that the drawings are not necessarilydrawn to scale.

Various modes of control may be utilized to regulate the output of apower converter. In PWM peak current mode control, the switch remains ONuntil the current in the switch reaches a regulation threshold. Once theregulation threshold is reached, the controller turns the switch off forthe remainder of the switching period. In general, the controllerregulates the output of the power converter by altering the duty ratioof the switch. The controller may alter the duty ratio by altering themagnitude of the regulation threshold. A greater regulation thresholdcorresponds to a longer ON time and a larger duty ratio for the switch.However, it should be appreciated that the regulation threshold isgenerally fixed for an individual switching cycle. For PWM peak currentmode control, the controller generally receives an analog signalrepresentative of the output of the power converter. In one example, thesignal received by the controller may convey how far away the sensedoutput of the power converter is from the desired quantity. Thecontroller then alters the duty ratio of the switch based on thereceived analog signal.

Another mode of control is known as ON/OFF control, which enables ordisables a switching cycle. When a cycle is enabled, the switch mayconduct current while the switch cannot conduct current during adisabled cycle. The controller produces a sequence of enabled anddisabled switching cycles to regulate the output of the power converter.For ON/OFF control, the controller generally receives a logic signalrepresentative of the output of the power converter. In one example, thesignal received by the controller may be a series of logic-level pulses,which would enable or disable the switch. In another example, the signalreceived by the controller may be a digital signal used for enabling ordisabling the switch.

In one type of ON/OFF control, the controller turns ON the switch for afixed ON time during an enabled cycle. In another type of ON/OFFcontrol, referred to as current limited ON/OFF control, the controllerturns ON the switch during an enabled cycle and turns OFF the switchonce the current in the switch reaches a current limit threshold. Ingeneral, utilizing an enable signal in the form of a logic state torepresent the output of the power converter may be beneficial, as theenable signal may be more noise immune than an analog signalrepresentative of the output. However, due to the enabling and disablingof cycles, the effective switching frequency of the power converter mayfall into the audible noise range. In addition, the root-mean-squared(RMS) current may be higher for power converters using ON/OFF controland as such the power converter may be less efficient.

As will be discussed, examples in accordance with the teachings of thepresent invention provide a current limited ON/OFF control scheme with avariable current limit threshold. With discussed examples, thecontroller receives an enable signal representative of the output of thepower converter. The enable signal includes a series of events, whichenable or disable the power switch. In one example, the controller turnson the power switch in response to an event of the enable signal andturns off the power switch when the current in the power switch reachesthe variable current limit threshold. The variable current limitthreshold varies in response to the time between successive events ofthe enable signal. Further, the variable current limit threshold mayvary in response to the time between events of the enable signal over arange of loads coupled to the output of the power converter. In oneexample, the variable current limit threshold may be a ramp signal andthe ramp signal along with the time between events of the enable signalmay be used to modulate the drive signal which controls the switching ofthe power switch to regulate the output of the power converter.

In one example, the variable current limit threshold increases at anincrease rate at the end of each ON time of the power switch for a fixedtime period or until the maximum current limit threshold is reached. Inanother example, the variable current limit increases with a fixedincrease amount in response to the end of the ON time of the powerswitch. The variable current limit threshold then decreases at adecrease rate until the current in the power switch reaches the currentlimit threshold or the variable current limit threshold reaches theminimum current limit threshold. As such, examples in accordance withthe teachings of the present invention may have increased efficiency andmay reduce the likelihood of producing audible noise while preservingthe benefits of a logic or digital enable signal representative of theoutput of the power converter.

To illustrate, FIG. 1 shows an example power converter 100 includinginput V_(IN) 102, an energy transfer element T1 104, a primary winding106 of the energy transfer element T1 104, a secondary winding 108 ofthe energy transfer element T1 104, a switch S1 110, input return 111, aclamp circuit 112, a rectifier D1 114, an output capacitor C1 116, anoutput return 117, a load 118, a sense circuit 120, an enable circuit122, and a controller 124. Controller 124 further includes a drivecircuit block 126 and a current limit threshold generator 128. In oneexample, enable circuit 122 and sense circuit 120 may also be includedin controller 124. FIG. 1 further illustrates an output voltage V_(O)130, an output current I_(O) 132, an output quantity U_(O) 134, afeedback signal 136, an enable signal U_(EN) 138, a switch current I_(D)140, a current sense signal 142, a drive signal 144, and a current limitthreshold signal U_(ILIM) _(_) _(TH) 148. The example switched modepower converter 100 illustrated in FIG. 1 is coupled in a flybackconfiguration, which is just one example of a switched mode powerconverter that may benefit from the teachings of the present invention.It is appreciated that other known topologies and configurations ofswitched mode power converter may also benefit from the teachings of thepresent invention.

In the illustrated example, the power converter 100 provides outputpower to a load 118 from an unregulated input V_(IN) 102. In oneexample, the input V_(IN) 102 is a rectified and filtered ac linevoltage. In another example, the input voltage V_(IN) 102 is a dc inputvoltage. The input V_(IN) 102 is coupled to the energy transfer elementT1 104. In some examples, the energy transfer element T1 104 may be acoupled inductor. In other examples, the energy transfer element T1 104may be transformer. In the example of FIG. 1, the energy transferelement T1 104 includes two windings, a primary winding 106 andsecondary winding 108. N_(P) and N_(S) are the number of turns for theprimary winding 106 and secondary winding 108, respectively. In theexample of FIG. 1, primary winding 106 may be considered an inputwinding, and secondary winding 108 may be considered an output winding.The primary winding 106 is further coupled to power switch S1 110, whichis then further coupled to the input return 111. In addition, the clampcircuit 112 is coupled across the primary winding 106 of the energytransfer element T1 104.

The secondary winding 108 of the energy transfer element T1 104 iscoupled to the rectifier D1 114. In the example illustrated in FIG. 1,the rectifier D1 114 is exemplified as a diode and the secondary winding108 is coupled to the anode of the diode. In some examples, therectifier D1 114 may be a transistor used as a synchronous rectifier.When a transistor is utilized as a synchronous rectifier, anothercontroller (referred to as a secondary controller) may be utilized tocontrol the turning ON and OFF of the transistor. In examples, theenable circuit 122 and/or sense circuit 120 may be included in thesecondary controller (not shown). As shown in the depicted example, theoutput capacitor C1 116 and the load 118 are coupled to the rectifier D1114. In the example of FIG. 1, both the output capacitor C1 116 and theload 118 are coupled to the cathode of the diode. An output is providedto the load 118 and may be provided as either an output voltage V_(O)130, output current I_(O) 132, or a combination of the two.

The power converter 100 further includes circuitry to regulate theoutput, which is exemplified as output quantity U_(O) 134. A sensecircuit 120 is coupled to sense the output quantity U_(O) 134 and toprovide feedback signal U_(FB) 136, which is representative of theoutput quantity U_(O) 134. Feedback signal U_(FB) 136 may be voltagesignal or a current signal. In one example, the sense circuit 120 maysense the output quantity from an additional winding included in theenergy transfer element T1 104 In another example, there may be agalvanic isolation (not shown) between the controller 124 and the enablecircuit 122 or between the enable circuit 122 and the sense circuit 120.The galvanic isolation could be implemented by using devices such as anopto-coupler, a capacitor or a magnetic coupling. In a further example,the sense circuit 120 may utilize a voltage divider to sense the outputquantity U_(O) 134 from the output of the power converter 100. Ingeneral, the output quantity U_(O) 134 is either an output voltage V_(O)130, output current I_(O) 132, or a combination of the two.

As shown in the depicted example, enable circuit 122 is coupled to sensecircuit 120 and receives feedback signal U_(FB) 136 representative ofthe output of power converter 100 from the sense circuit 120. Enablesignal U_(EN) 138 may be a voltage signal or a current signal. In oneexample, enable signal U_(EN) 138 is also representative of the outputof the power converter 100 and provides information to the controller124 to enable or disable the power switch S1 110. Further, the enablesignal U_(EN) 138 may include one or more enable events, which cause thepower switch S1 110 to be enabled (or disabled). For example, the powerswitch S1 110 may be enabled when an enable event in enable signalU_(EN) 138 is received. In one example, the enable circuit 122 outputsenable signal U_(EN) 138, which in one example is a rectangular pulsewaveform with varying lengths of logic high and logic low sections. Inanother example, the enable signal U_(EN) 138 may be a logic or digitalsignal. An enable event in enable signal U_(EN) 138 may be a pulse or aseries of pulses that enable (or disable) the power switch S1 110. Inanother example, an enable event in enable signal U_(EN) 138 may be atransition from one logic state to another logic state, which enables(or disables) the power switch S1 110. In a further example, enablesignal U_(EN) 138 may be an analog signal, and an enable event may beindicated with enable signal U_(EN) 138 crossing of a threshold value.

Controller 124 is coupled to the enable circuit 122 and receives enablesignal U_(EN) 138 from the enable circuit 122. The controller 124further includes terminals for receiving the current sense signal 142and for providing the drive signal 144 to power switch S1 110. Thecurrent sense signal 142 may be representative of the switch currentI_(D) 140 in power switch S1 110. Current sense signal 142 may be avoltage signal or a current signal. In addition, the controller 124provides drive signal 144 to the power switch S1 110 to control variousswitching parameters to control the transfer of energy from the input ofpower converter 100 to the output of power converter 100. Examples ofsuch parameters may include switching frequency, switching period, dutycycle, or respective ON and OFF times of the power switch S1 110.

As illustrated in example depicted in FIG. 1, the controller 124includes drive circuit 126 and current limit threshold generator 128.The drive circuit 126 is coupled to receive the enable signal U_(EN)138. In one example, drive circuit 126 outputs drive signal 144 inresponse to the enable signal U_(EN) 138. In some examples, drivecircuit 126 further receives current sense signal 142 and outputs drivesignal 144 in further response to the current sense signal 142. Currentlimit threshold generator 128 is coupled to receive the drive signal 144from the drive circuit 126 and further outputs the current limitthreshold signal U_(ILIM) _(_) _(TH) 148 to the drive circuit 126. Inone example, current limit threshold generator 128 is coupled to varythe current limit threshold signal U_(ILIM) _(_) _(TH) 148 in responseto a time between the enable events of the enable signal U_(EN) 138. Inone example, the current limit threshold signal U_(ILIM) _(_) _(TH) 148may be a ramp signal and the ramp signal along with the time betweenenable events may be used to modulate the drive signal 144 to regulatethe output of the power converter.

For instance, in one example, the current limit threshold generator 128is coupled to increase, within a current limit threshold range, thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 148 at a increaserate during a fixed time period after an end of each ON time of thepower switch S1 110. In the example, after the fixed time period afterthe end of each ON time of the power switch S1 110, the current limitthreshold generator 128 is coupled to decrease the current limitthreshold signal U_(ILIM) _(_) _(TH) 148, within the current limitthreshold range, at a decrease rate until the current through powerswitch S1 110 reaches the current limit threshold. In one example, thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 148 may be a voltagesignal or a current signal. As illustrated, the drive circuit 126 alsooutputs drive signal 144 in response to the current limit thresholdsignal U_(ILIM) _(_) _(TH) 148.

In the example of FIG. 1, input voltage V_(IN) 102 is positive withrespect to input return 111, and output voltage V_(O) 130 is positivewith respect to output return 117. In the example illustrated in FIG. 1,the input return 111 is galvanically isolated from the output return117. In other words, a dc voltage applied between input return 111 andoutput return 117 will produce substantially zero current. Therefore,circuits electrically coupled to the primary winding 106 aregalvanically isolated from circuits electrically coupled to thesecondary winding 108. For example, galvanic isolation could beimplemented by using an opto-coupler, a capacitive coupler or a magneticcoupler between the controller 124 and the enable circuit 122 or betweenthe enable circuit 122 and the sense circuit 120.

In one example, the power converter 100 of FIG. 1 provides regulatedoutput power to the load 118 from an unregulated input V_(IN) 102. Thepower converter 100 utilizes the energy transfer element T1 104 totransfer energy between the primary 106 and secondary 108 windings. Theclamp circuit 112 is coupled to the primary winding 106 of the energytransfer element T1 104 to limit the maximum voltage on the power switchS1 110. In the example power converter 100 shown in FIG. 1, the clampcircuit 112 limits the voltage spike caused by the leakage inductance ofthe primary winding 106 after the power switch S1 110 has turned OFF.Power switch S1 110 is opened and closed in response to the drive signal144 received from the controller 124 to control the transfer of energyfrom the input of the power converter 100 to the output of powerconverter 100. It is generally understood that a switch that is closedmay conduct current and is considered on, while a switch that is opencannot conduct current and is considered off. In the example of FIG. 1,power switch S1 110 controls a current I_(D) 140 in response tocontroller 124 to meet a specified performance of the power converter100. In some examples, the power switch S1 110 may be a transistor andthe controller 124 may include integrated circuits and/or discreteelectrical components. In one example, controller 124 and power switchS1 110 are included together in a single integrated circuit. In oneexample, the integrated circuit is a monolithic integrated circuit. Inanother example, the integrated circuit is a hybrid integrated circuit.

The operation of power switch S1 110 also produces a time varyingvoltage V_(P) across the primary winding 106. By transformer action, ascaled replica of the voltage V_(P) is produced across the secondarywinding 108, the scale factor being the ratio that is the number ofturns N_(S) of secondary winding 108 divided by the number of turnsN_(P) of primary winding 106. The switching of power switch S1 110 alsoproduces a pulsating current at the rectifier D1 114. The current inrectifier D1 114 is filtered by output capacitor C1 116 to produce asubstantially constant output voltage V_(O) 130, output current I_(O)132, or a combination of the two at the load 118.

In the illustrated example, sense circuit 120 senses the output quantityU_(O) 134 to provide the feedback signal U_(FB) 136 representative ofthe output of power converter 100 to the enable circuit 122. The enablecircuit 122 receives the feedback signal U_(FB) 136 and produces anenable signal U_(EN) 138. The enable signal U_(EN) 138 is representativeof the output of the power converter 100 and provides information to thecontroller 124 (using enable events) to enable or disable the powerswitch S1 110. Further, the time between enable events of the enablesignal U_(EN) 138 is responsive to the power converter output. Inexamples, an enable event may be generated when the output quantityU_(O) 134 or feedback signal U_(FB) 136 falls below a threshold. In oneexample, the enable signal U_(EN) 138 may utilize a pulse (the enablesignal increases to a logic high value and decreases to a logic lowvalue) as the enable event to control the power switch S1 110.

In the example of FIG. 1, the controller 124 receives the enable signalU_(EN) 138 and also receives the current sense signal 142, which isrepresentative of the sensed switch current I_(D) 140 in the powerswitch S1 110. The switch current I_(D) 140 may be sensed in a varietyof ways, such as for example, the voltage across a discrete resistor orthe voltage across the transistor when the transistor is conducting. Thecontroller 124 outputs drive signal 144 to operate the power switch S1110 in response to various inputs to substantially regulate the outputquantity U_(O) 134 to the desired value. With the use of the sensecircuit 120, enable circuit 122, and the controller 124, the output ofthe power converter 100 is regulated in a closed loop in accordance withthe teachings of the present invention.

As shown in the depicted example, controller 124 further includes drivecircuit 126, which receives the enable signal U_(EN) 138 and currentsense signal 142. Drive circuit 126 outputs the drive signal 144 tocontrol switching the power switch S1 110 in response to the enablesignal U_(EN) 138 and current sense signal 142 to control the transferof energy from the input of power converter 100 to the output of powerconverter 100. In one example, drive circuit 126 turns ON the powerswitch S1 110 in response to an enable event. In one example, drivecircuit 126 turns ON the power switch S1 110 when the enable signalU_(EN) 138 pulses to a logic high value. In one example, drive circuit126 turns OFF the power switch S1 110 when the switch current I_(D) 140represented with the current sense signal 142 reaches the current limitthreshold signal U_(ILIM) _(_) _(TH) 148. In one example, the drivesignal 144 is a rectangular pulse waveform with varying lengths of logichigh and logic low sections. Drive signal 144 may be a voltage signal ora current signal. In one example, the power switch S1 110 is ON when thedrive signal 144 is logic high and the power switch S1 110 is OFF whenthe drive signal 144 is logic low.

As shown in the depicted example, the drive signal 144 is also coupledto be received by the current limit threshold generator 128. In oneexample, the current limit threshold generator 128 generates the currentlimit threshold signal U_(ILIM) _(_) _(TH) 148 in response to the drivesignal 144. As will be further discussed, the current limit thresholdsignal U_(ILIM) _(_) _(TH) 148 increases, within a current limitthreshold range, at an increase rate for a fixed time period after theend of the ON time of the power switch S1 110. In other words, thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 148 increases by afixed amount, within the current limit threshold range, at the end ofthe ON time of the power switch S1 110. Thus, in one example the currentlimit threshold signal U_(ILIM) _(_) _(TH) 148 does not increase beyonda maximum current limit threshold. After the fixed time period, thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 148 decreases, withinthe current limit threshold range, at a decrease rate. In one example,the current limit threshold signal U_(ILIM) _(_) _(TH) 148 decreasesuntil the switch current I_(D) 140 indicated by the current sense signal142 reaches the current limit threshold signal U_(ILIM) _(_) _(TH) 148or until the current limit threshold signal U_(ILIM) _(_) _(TH) 148reaches a minimum current limit threshold.

As mentioned above, the drive signal 144 is generated in response to theenable signal U_(EN) 138. In one example, current limit thresholdgenerator 128 therefore also generates the current limit thresholdsignal U_(ILIM) _(_) _(TH) 148 in response to the enable signal U_(EN)138. In particular, the current limit threshold signal U_(ILIM) _(_)_(TH) 148 is responsive to the time between enable events of the enablesignal U_(EN) 138 over a range of loads coupled to the output of thepower converter 100. In another example, the current limit thresholdsignal U_(ILIM) _(_) _(TH) 148 may be a ramp signal and the ramp signalalong with the time between enable events may be used to modulate thedrive signal 144 to regulate the output of the power converter. As such,examples in accordance with the teachings of the present invention mayhave increased efficiency and may reduce the likelihood of producingaudible noise while preserving the benefits of a logic or digital enablesignal representative of the output of the power converter 100.

FIG. 2A illustrates an example graph 200 illustrating examplerelationships of the current limit threshold I_(LIM) 250 decreasing overtime in accordance with the teachings of the present invention. Inparticular, graph 200 illustrates a first relationship 252 of thecurrent limit threshold I_(LIM) 250, a second relationship 254 of thecurrent limit threshold I_(LIM) 250, a maximum current limit thresholdI_(TH) _(_) _(MAX) 256, a minimum current limit threshold I_(TH) _(_)_(MIN) 258, a time t₁ 260, and a time t₂ 262. Further illustrated is a100% current limit 264 that corresponds to the highest value of thecurrent limit threshold I_(LIM) 250, which the switch current I_(D) 140may reach since the current limit threshold I_(LIM) 250 is variable andbegins decreasing a fixed time period after the power switch S1110 turnsoff. In one example, the fixed time period is substantially zero. Inaddition, current limit threshold range 265 is the range of valuesbetween the minimum current limit threshold I_(TH) _(_) _(MIN) 258 tothe maximum current limit threshold I_(TH) _(_) _(MAX) 256 which thecurrent limit threshold generator may vary the current limit thresholdI_(LIM) 250.

As shown in the example first relationship 252, the current limitthreshold I_(LIM) 250 decreases, within a current limit threshold range265, with a first decrease rate from the maximum current limit thresholdI_(E) _(_) _(MAX) 256 to the minimum current limit threshold I_(TH) _(_)_(MIN) 258. The current limit threshold I_(LIM) 250 reaches the minimumcurrent limit threshold I_(TH) _(_) _(MIN) 258 at time t₁ 260. Once thecurrent limit threshold I_(LIM) 250 decreases to the minimum currentlimit threshold I_(TH) _(_) _(MIN) 258, the current limit thresholdI_(LIM) 250 stops decreasing and is substantially equal to the minimumcurrent limit threshold I_(TH) _(_) _(MIN) 258.

As shown in the example second relationship 254, the current limitthreshold I_(LIM) 250 decreases, within the current limit thresholdrange 265, with a second decrease rate from the maximum current limitthreshold I_(TH) _(_) _(MAX) 256 to the minimum current limit thresholdI_(TH) _(_) _(MIN) 258. The current limit threshold I_(LIM) 250 reachesthe minimum current limit threshold I_(TH) _(_) _(MIN) 258 at time t₂262. Once the minimum current limit threshold I_(TH) _(_) _(MIN) 258 isreached, the current limit threshold I_(LIM) 250 stops decreasing and issubstantially equal to the minimum current limit threshold I_(TH) _(_)_(MIN) 258. The first relationship 252 and the second relationship 254illustrate the current limit threshold I_(LIM) 250 substantiallylinearly decreasing with respect to time. However, examples may alsoinclude relationships in which the current limit threshold I_(LIM) 250is non-linear and/or monotonic. For example, the relationship may bequadratic, exponential, or piecewise linear. Further examples may alsoinclude relationships in which the current limit threshold I_(LIM) 250may include a series of decreasing steps. The series of decreasing stepsmay be substantially linearly decreasing or non-linearly decreasing.

In one example controller may select the first relationship 252 or thesecond relationship 254 to utilize for decreasing the current limitthreshold 250. For instance, in one example the controller may selectthe first relationship 252 or the second relationship 254 in response tothe input voltage V_(IN) 102 of the power converter 100. The currentlimit threshold I_(LIM) 250 illustrated in FIG. 2A may be utilized tovary the current limit threshold signal U_(ILIM) _(_) _(TH) 148. Inparticular, the relationships shown in FIG. 2A may be utilized todetermine how the current limit threshold signal U_(ILIM) _(_) _(TH) 148responds to the enable signal U_(EN) 138 and/or drive signal 144.

FIG. 2B illustrates another example graph 201 illustrating therelationship of the current limit threshold signal U_(ILIM) _(_) _(TH)248 over time. In the depicted example, the current limit thresholdsignal U_(ILIM) _(_) _(TH) 248, switch current I_(D) 240, and enablesignal U_(EN) 238 are respective examples of current limit thresholdsignal U_(ILIM) _(_) _(TH) 148, switch current I_(D) 140, and enablesignal U_(EN) 138 discussed above with respect to FIG. 1. In addition,the first relationship 252 may be utilized to determine how the currentlimit threshold signal U_(ILIM) _(_) _(TH) 248 responds to the enablesignal U_(EN) 238.

As illustrated in the depicted example, the current limit thresholdsignal U_(ILIM) _(_) _(TH) 248 decreases, within the current limitthreshold range 265, from the maximum current limit threshold I_(TH)_(_) _(MAX) 256. When an enable event occurs in enable signal U_(EN)238, which is indicated in the example with the enable signal U_(EN) 238pulsing to a logic high value (in other words, an enable pulse isreceived), the power switch S1 110 is turned ON and the switch currentI_(D) 240 begins to increase. When the switch current I_(D) 240 reachesthe current limit threshold signal U_(ILIM) _(_) _(TH) 248, the powerswitch S1 110 is turned OFF and the switch current I_(D) 240 falls tozero. Further, the current limit threshold signal U_(ILIM) _(_) _(TH)248 increases in response to the power switch S1 110 being turned OFF.In one example, the current limit threshold signal U_(ILIM) _(_) _(TH)248 increases to the maximum current limit threshold I_(TH) _(_) _(MAX)256. However, in other examples the current limit threshold signalU_(ILIM) _(_) _(TH) 248 increases by a fixed amount within the currentlimit threshold range 265. In the example illustrated in FIG. 2B, oncethe current limit threshold signal U_(TLIM) _(_) _(TH) 248 reaches themaximum current limit threshold I_(TH) _(_) _(MAX) 256, the currentlimit threshold signal U_(TLIM) _(_) _(TH) 248 begins to decrease again.In the example shown in FIG. 2B, another enable pulse is not receivedfrom the enable signal U_(EN) 238 and as such the current limitthreshold signal U_(TLIM) _(_) _(TH) 248 decreases, within the currentlimit threshold range 265, to the minimum current limit threshold I_(TH)_(_) _(MIN) 258. FIG. 2B also illustrates a dashed line 253 that showshow the current limit threshold signal U_(ILIM) _(_) _(TH) 248 wouldhave decreased, within the current limit threshold range 265, if theenable event in enable signal U_(EN) 238 had not been received and thepower switch S1 110 had therefore had not been turned ON.

FIG. 3 illustrates an example controller 300, which in one example maybe controller 100 of FIG. 1. It should be appreciated that similarlynamed and numbered elements referenced below are coupled and function asdescribed above. As mentioned above, drive circuit 326 is coupled toreceive the enable signal U_(EN) 338, current sense signal 342 and thecurrent limit threshold signal U_(TLIM) _(_) _(TH) 348. In theillustrated example, drive circuit 326 is shown including latch 366,which in the illustrated example is coupled to be reset by comparator368. In the example, latch 366 is coupled to receive the enable signalU_(EN) 338 at its S-input while the output of comparator is coupled tothe R-input of latch 366. The drive signal 344 is output from the latch366. As shown, the drive signal 344 is output from the Q-output of latch366. As will be further discussed, the Q-output of the latch 366 islogic high if the enable signal U_(EN) 338 is logic high. In oneexample, the enable signal U_(EN) 338 is a rectangular pulse waveform,which transitions to a logic high value and quickly falls to a logic lowvalue. In one example, the occurrence of a logic high pulse of theenable signal U_(EN) 338 may be referred to as an enable event. When anenable event is received at the S-input of latch 366, the drive signal344 transitions to a logic high value. Drive signal 344 transitions to alogic low value when a logic high value is received at the R-input oflatch 366.

As shown in the depicted example, comparator 368 is coupled to receivethe current sense signal 342 and the current limit threshold signalU_(ILIM) _(_) _(TH) 348. In the example shown in FIG. 3, the currentsense signal 342 is received at the non-inverting input of comparator368 while the current limit threshold signal U_(ILIM) _(_) _(TH) 348 isreceived at the inverting input of comparator 368. Drive signal 344transitions to a logic low value when the current sense signal 342reaches the current limit threshold signal U_(ILIM) _(_) _(TH) 348. Asmentioned above, in one example the current sense signal 342 isrepresentative of the switch current I_(D) 140. As such, the drivesignal 344 transitions to a logic low value when the switch currentI_(D) 140 represented with current sense signal 342 reaches the currentlimit threshold signal U_(ILIM) _(_) _(TH) 348. In one example, thedrive signal 344 is a rectangular pulse waveform with varying lengths oflogic high and logic low sections. In one example, the length of timethat the drive signal 344 is logic high corresponds to the ON time(t_(ON)) of the power switch S1 110 and the length of time the drivesignal 344 is logic low corresponds to the OFF time (t_(OFF)) of thepower switch S1 110. In addition, the length of time between risingedges of the drive signal 344 may be referred to as the switching periodT_(S).

Current limit threshold generator 328 is coupled to receive the drivesignal 344 from the drive circuit 326. In the example depicted in FIG.3, a monostable multivibrator 370 is coupled to receive the drive signal344 from latch 366. In one example, the monostable multivibrator 370generates a pulse with a fixed time period (in other words, the pulse islogic high for a fixed time period) in response to an edge of the drivesignal 344. In one example, the monostable multivibrator 370 generates apulse with a fixed time period in response to the falling edge of thedrive signal 344. In other words, the monostable multivibrator 370generates a pulse with a fixed time period at the end of the ON time(t_(ON)) of the power switch. The output of the monostable multivibrator370 is referred to as the one shot signal OS 390.

As illustrated, AND gate 386 and inverter 389 are coupled to receive theone shot signal OS 390 from the monostable multivibrator 370. Theinverter 389 is further coupled to AND gate 388 such that AND gate 388receives the inverted one shot signal OS 391. AND gates 386 and 388output the charge signal CHG 392 and discharge signal DIS 394(respectively) to control switching of switches S2 374 and S3 378. Oneend of switch S2 374 is coupled to current source 372 while the otherend of switch S2 374 is coupled to one end of switch S3 378. The otherend of switch S3 378 is coupled to current source 376. One end ofcapacitor 380 is coupled to a node between switch S2 374 and switch S3378. As illustrated, the voltage across capacitor 380 is output from thecurrent limit threshold generator 328 as current limit threshold signalU_(ILIM) _(_) _(TH) 348.

In one example, current source 376 may be a controlled current source.As illustrated in FIG. 3, current source 376 may be coupled to receiveselect signal SELECT 396. Select signal 396 may be utilized to selectthe magnitude of I_(DIS) of current source 376. Referring back to theexamples depicted FIG. 2A, the first or second relationship 252 or 254may be selected for the current limit threshold 250 in response to theinput voltage V_(IN) 102 of the power converter 100. As will be furtherdiscussed, the magnitude of I_(DIS) of current source 376 affects thedischarge rate of the capacitor 380. As such, the select signal 396 mayselect the magnitude of I_(DIS) of current source 376 in response to theinput voltage V_(IN) 102 of the power converter in accordance with theteachings of the present invention. For instance, in one example, selectsignal 396 may set a first magnitude for I_(DIS) for a first inputvoltage value for V_(IN) 102 and select signal 396 may set a secondmagnitude for I_(DIS) for a second input voltage value for V_(IN) 102 inaccordance with the teachings of the present invention. In other words,in one example, a plurality of different I_(DIS) magnitudes for currentsource 376 may be selected in response to the input voltage V_(IN), asrepresented with select signal 396 in FIG. 3, in accordance with theteachings of the present invention. In one example, a lower inputvoltage V_(IN) 102 may correspond to a larger magnitude for I_(DIS). Inanother example, a first magnitude of I_(DIS) for the current source 376may be selected for a first range of V_(IN) 102 and a second magnitudeof I_(DIS) for the current source 376 may be selected for second rangeof V_(IN) 102. In one example, there could be several ranges of V_(IN)102 and corresponding magnitudes of I_(DIS) for the current source 376.The ranges of V_(IN) 102 in one example, could correspond to ac voltageranges needed to operate in different geographies; 100 VAC−15% to 115VAC+15% for Japan and the U.S., 230 VAC+/−15% for Europe, etc.

Both comparators 382 and 384 are coupled to capacitor 380 to receive thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 348. As illustrated,comparator 382 receives the current limit threshold signal U_(ILIM) _(_)_(TH) 348 at its non-inverting input while comparator 384 receives thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 348 at its invertinginput. Comparator 382 also receives the maximum current limit thresholdU_(TH) _(_) _(MAX) 356 at its inverting input while the comparator 384receives the minimum current limit threshold U_(TH) _(_) _(MIN) 358 atits non-inverting input. In the illustrated example, AND gates 386 and388 are coupled to receive the inverted outputs of comparator 382 and384, respectively, as illustrated by the small circle at one of theinputs for both AND gates 386 and 388.

When the one shot signal OS 390 transitions to a logic high value, thecharging signal CHG transitions to a logic high value and switch S2 374is closed. In addition, the discharge signal DIS 394 transitions to alogic low value and opens switch S3 378. As such, the capacitor 380 ischarged by current source 372 with current I_(C). In one example, theamount at which the voltage (i.e., the current limit threshold signalU_(TLIM) _(_) _(TH) 348) across capacitor 380 increases is proportionalto the magnitude of current I_(C) provided by current source 372 and theamount of time the one shot signal OS 390 is logic high (i.e., the fixedtime period). In particular, the amount which the current limitthreshold signal U_(ILIM) _(_) _(TH) 348 increases is substantiallyequal to the product of the magnitude of current I_(C) and the fixedtime period divided by the capacitance of capacitor 380. Ormathematically: ΔU_(ILIM) _(_) _(TH)≈I_(C)t_(FIXED)/C. Or in otherwords, the increase rate of the current limit threshold signal U_(ILIM)_(_) _(TH) 380 is proportional to the magnitude of current I_(C) and thecapacitance of capacitor 380.

Charging signal CHG 392 transitions to logic low if the one shot signalOS 390 transitions to a logic low value or the voltage across capacitor380 (i.e., current limit threshold signal U_(TLIM) _(_) _(TH) 348)reaches the maximum current limit threshold U_(TH) _(_) _(MAX) 356. Whenthe charging signal CHG 392 is a logic low value, the switch S2 374opens and capacitor 380 is no longer charged by current source 372.

When the inverted one shot signal OS 391 transitions to the logic highvalue, the discharging signal DIS 394 transitions to a logic high valueand closes switch S3 378. As such, the capacitor 380 is discharged bythe current source 376 with current I_(DIS). The decrease rate of thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 348 is proportionalto the magnitude of current I_(DIS) and the capacitance of capacitor380. In one example, the magnitude of the increase rate is greater thanthe magnitude of the decrease rate.

Discharging signal DIS 394 transitions to logic low if the inverted oneshot signal OS 391 transitions to a logic low value or if the voltageacross capacitor 380 (i.e., current limit threshold signal U_(TLIM) _(_)_(TH) 348) reaches the minimum current limit threshold U_(TH) _(_)_(MIN) 358. When the discharging signal DIS 394 is a logic low value,the switch S3 378 opens and capacitor 380 is no longer discharged bycurrent source 376.

Referring to FIG. 4, a timing diagram 400 that illustrates variousexample waveforms of signals of the controller 300 of FIG. 3 is shown inaccordance with the teachings of the present invention. It should beappreciated that similarly named and numbered elements referenced beloware coupled and function as described above. In the example depicted inFIG. 4, the current limit threshold range 465 is the range of valuesbetween the minimum current limit threshold U_(TH) _(_) _(MIN) 458 tothe maximum current limit threshold U_(TH) _(_) _(MAX) 456 which thecurrent limit threshold generator 328 may vary the current limitthreshold signal U_(TLIM) _(_) _(TH) 448. The waveforms described in thetiming diagram 400 illustrate that, in one example, a fixed increaseamount of the current limit threshold signal U_(ILIM) _(_) _(TH) 448 foreach fixed time period in which one shot signal OS 490 is a logic highis less than the current limit threshold range 465 such that a pluralityof consecutive switching cycles of the power switch S1 110 occur beforethe current limit threshold generator 328 can vary the current limitthreshold signal U_(ILIM) _(_) _(TH) 448 from the minimum current limitthreshold U_(TH) _(_) _(MIN) 458 up to the maximum current limitthreshold U_(TH) _(_) _(MAX) 456. For instance, in the example shown inFIG. 4, three consecutive switching periods are utilized to vary thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 448 from the minimumcurrent limit threshold U_(TH) _(_) _(MIN) 458 to the maximum currentlimit threshold U_(TH) _(_) _(MAX) 456. In other examples (not shown),the current limit threshold signal U_(TLIM) _(_) _(TH) 448 for eachfixed time period could vary from the minimum current limit thresholdU_(TH) _(_) _(MIN) 458 up to the maximum current limit threshold U_(TH)_(_) _(MAX) 456.

To illustrate, at the beginning of switching period T₁, the currentlimit threshold signal U_(ILIM) _(_) _(TH) 448 (shown as the boldedline) is substantially equal to the minimum current limit thresholdU_(TH) _(_) _(MIN) 458. An enable event is received (as shown by theenable signal U_(EN) 438 transitioning to a logic high value) by thelatch 366 and the drive signal 444 transitions to a logic high value,which therefore turns ON the power switch S1 110. The current sensesignal 442 (representative of the switch current I_(D) 140) begins toincrease from zero. The rate at which the switch current I_(D) 140 andcurrent sense signal 442 increases is proportional to the input voltageV_(IN) of the power converter. When the current sense signal 442 reachesthe current limit threshold signal U_(TLIM) _(_) _(TH) 448, the outputof comparator 368 transitions to a logic high value, which resets latch366 causing the drive signal 444 to transition to a logic low value andthe power switch S1 110 is turned OFF. As shown, the time in which thedrive signal 444 is logic high is referred to as the ON time (t_(ON)) ofthe power switch S1 110 and the time in which the drive signal 444 islogic low may be referred to as the OFF time (t_(OFF)) of the powerswitch S1 110. Once the power switch is turned OFF, the current sensesignal 442 falls to zero.

At the falling edge of the drive signal 444 during switching period T₁,the one shot signal OS 490 transitions to a logic high value for a fixedtime period. During switching period T₁, the value of the current limitthreshold signal U_(TLIM) _(_) _(TH) 448 is less than the maximumcurrent limit threshold U_(TH) _(_) _(MAX) 456 for the entirety of thefixed time period. As such, the output of comparator 382 is logic lowand the charge signal CHG 392 is logic high for as long as the one shotsignal OS 490 is logic high. Switch S2 374 is closed and the capacitoris charged by current source 372. As a result, the current limitthreshold signal U_(TLIM) _(_) _(TH) 448 increases for as long as thecharge signal CHG 392 is logic high.

As illustrated the current limit threshold signal U_(TLIM) _(_) _(TH)448 increases, within the current limit threshold range 465, with aincrease rate during a fixed time period after the end of the ON timet_(ON) of the power switch. Referring back to FIG. 3, the increase ratefor the current limit threshold signal U_(ILIM) _(_) _(TH) 448 issubstantially proportional to the current I_(C) provided by currentsource 372 and the capacitance of capacitor 380. In particular, themaximum amount which the current limit threshold signal U_(ILIM) _(_)_(TH) 448 may increase by is substantially equal to the product of themagnitude of current I_(C) and the fixed time period divided by thecapacitance of capacitor 380. As shown in the illustrated example, themaximum amount that the current limit threshold signal U_(ILIM) _(_)_(TH) 448 may increase is less than the current limit threshold range465.

The inverted one shot signal OS 491 transitions to a logic high value asthe one shot signal OS 490 transitions to a logic low value at the endof the fixed time period. The output of comparator 384 is logic low aslong as the value of the current limit threshold signal U_(ILIM) _(_)_(TH) 448 is greater than the minimum current limit threshold U_(TH)_(_) _(MIN) 458. As such, the discharge signal DIS 494 output from ANDgate 388 is logic high until the inverted one shot signal OS 491transitions to a logic low value or the value of the current limitthreshold signal U_(ILIM) _(_) _(TH) 448 reaches the minimum currentlimit threshold U_(TH) _(_) _(MIN) 458. As illustrated, the currentlimit threshold signal U_(TLIM) _(_) _(TH) 448 decreases with a decreaserate until the current sense signal 442 reaches the current limitthreshold signal U_(TLIM) _(_) _(TH) 448. Referring back to FIG. 3, thedecrease rate is substantially proportional to the current I_(DIS)provided by current source 376 and the capacitance of capacitor 380.

At the start of switching period T_(z), the current limit thresholdsignal U_(TLIM) _(_) _(TH) 448 is still decreasing with the decreaserate. Another enable event is received (as shown by the enable signalU_(EN) 438 transitioning to a logic high value at the start of switchingperiod T₂), which sets the latch 366 and causes the drive signal 444 totransition to a logic high value, which turns ON power switch S1 110.When the current sense signal 442 reaches the current limit thresholdsignal U_(ILIM) _(_) _(TH) 448 (which is still decreasing), the outputof comparator 368 transitions to a logic high value, which resets latch366 and causes the drive signal 444 to transition to a logic low value,which turns OFF the power switch S1 110.

Continuing with the example depicted in FIG. 4, the one shot signal OS490 transitions to a logic high value at the end of the ON time t_(ON)during switching period T₂. Similar to switching period T₁, duringswitching period T₂ the value of the current limit threshold signalU_(ILIM) _(_) _(TH) 448 is less than the maximum current limit thresholdU_(TH) _(_) _(MAX) 456 for the entirety of the fixed time period atwhich the one shot signal OS 490 is logic high. As such, the chargesignal CHG 492 is logic high for the entirety of the fixed time periodand transitions to a logic low value when the one shot signal OS 490transitions to a logic low value after the fixed time period. Or inother words, the charge signal CHG 492 substantially follows the oneshot signal OS 490. At the end of the fixed time period (i.e., one shotsignal OS 490 has transitioned to a logic low value and the inverted oneshot signal OS 491 has transitioned to a logic high value), the currentlimit threshold signal U_(ILIM) _(_) _(TH) 448 decreases with thedecrease rate until the minimum current limit threshold U_(TH) _(_)_(MIN) 458 or the current sense signal 442 reaches the current limitthreshold signal U_(ILIM) _(_) _(TH) 448. In the example shown, thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 448 decreases untilthe current sense signal 442 reaches the current limit threshold signalU_(ILIM) _(_) _(TH) 448.

As shown in the depicted example, another enable event is received atthe start of switching period T₃ and the current limit threshold signalU_(ILIM) _(_) _(TH) 448 is still decreasing with the decrease rate.Switching period T₃ is similar to switching period T₂. However, at theend of the fixed time period of the one shot signal OS 490, the currentlimit threshold signal U_(TLIM) _(_) _(TH) 448 has just reached themaximum current limit threshold U_(TH) _(_) _(MAX) 456. At the end ofthe fixed time period, the current limit threshold signal U_(TLIM) _(_)_(TH) 448 begins decreasing.

Another enable event is received at the start of switching period T₄ andthe current limit threshold signal U_(ILIM) _(_) _(TH) 448 is stilldecreasing with the decrease rate. The drive signal 444 transitions to alogic high value and the power switch is turned ON. When the currentsense signal 442 reaches the current limit threshold signal U_(ILIM)_(_) _(TH) 448 the output of comparator 368 transitions to a logic highvalue and the drive signal 444 transitions to a logic low value and thepower switch is turned OFF.

The one shot signal OS 490 transitions to a logic high value at the endof the ON time t_(ON) during switching period T₄. At the end of the ONtime t_(ON) during switching period T₄, the value of the current limitthreshold signal U_(TLIM) _(_) _(TH) 448 is less than the maximumcurrent limit threshold U_(TH) _(_) _(MAX) 456. As such the output ofcomparator 382 is logic low and the charge signal CHG 492 is logic high.Switch S2 374 is turned ON and the current limit threshold signalU_(ILIM) _(_) _(TH) 448 begins to increase.

However, unlike switching periods T₁, T₂, and T₃, the current limitthreshold signal U_(TLIM) _(_) _(TH) 448 reaches the maximum currentlimit threshold U_(TH) _(_) _(MAX) 456 before the end of the fixed timeperiod (i.e., the one shot signal OS 490 is still logic high). When thecurrent limit threshold signal U_(TLIM) _(_) _(TH) 448 reaches themaximum current limit threshold U_(TH) _(_) _(MAX) 456, the output ofcomparator 382 is logic high and the charge signal CHG 492 transitionsto a logic low value. As such, switch S2 374 is turned OFF and thecapacitor 380 is no longer charged by current source 372. As will befurther illustrated in FIG. 5, the current limit threshold signalU_(ILIM) _(_) _(TH) 448 remains substantially equal to the maximumcurrent limit threshold U_(TH) _(_) _(MAX) 456 for the remainder of thefixed time period in which one shot signal OS 490 is a logic high value.Or in other words, the current limit threshold signal U_(TLIM) _(_)_(TH) 448 is substantially equal to the maximum current limit thresholdU_(TH) _(_) _(MAX) 456 until the one shot signal OS 490 transitions to alogic low value.

At the end of the fixed time period, the inverted one shot signal OS 491transitions to a logic high value. The output of comparator 384 is logiclow since the value of the current limit threshold signal U_(TLIM) _(_)_(TH) 448 is greater than the minimum current limit threshold U_(TH)_(_) _(MIN) 458. As a result the discharge signal DIS 494 is logic highand the switch S3 378 is closed and the current limit threshold signalU_(ILIM) _(_) _(TH) 448 decreases until the current sense signal 442reaches the current limit threshold signal U_(ILIM) _(_) _(TH) 448 orthe current limit threshold signal U_(ILIM) _(_) _(TH) 448 reaches theminimum current limit threshold U_(TH) _(_) _(MIN) 458. In the exampleshown in FIG. 4, the waveforms shown for switching periods T₅, T₆, T₇,and T₈ are similar to waveforms described with respect to switchingperiod T₄ discussed above.

FIG. 5 is a timing diagram illustrating in increased detail variousexample waveforms of signals shown in FIG. 4 in accordance with theteachings of the present invention. In particular, the timing diagram500 illustrates an example in which the current limit threshold signalU_(TLIM) _(_) _(TH) 548 is clamped within the current limit thresholdrange at the maximum current limit threshold U_(TH) _(_) _(MAX) 556. Atthe end of the ON time of the power switch S1 100 (as shown by the drivesignal 544 transitioning to the logic low value), the one shot signal OS590 transitions to a logic high value. As mentioned above, the length oftime at which the one shot signal OS 590 is logic high may be referredto as the fixed time period. At the beginning of the fixed time period,the current limit threshold signal U_(TLIM) _(_) _(TH) 548 is less thanthe maximum current limit threshold U_(TH) _(_) _(MAX) 556 and theoutput of comparator 382 is logic low. As such, the charge signal CHG592 transitions to a logic high value when the one shot signal OS 590transitions to a logic high value. The switch S2 374 is closed and thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 548 increases.

However, the current limit threshold signal U_(ILIM) _(_) _(TH) 548reaches the maximum current limit threshold U_(TH) _(_) _(MAX) 556before the end of the fixed time period of the one shot signal OS 590.The output of comparator 382 transitions to a logic high value and as aresult the charge signal CHG 592 transitions to a logic low value. Thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 548 is substantiallyequal to the maximum current limit threshold U_(TH) _(_) _(MAX) 556 forthe remainder of the fixed time period of the one shot signal OS 590.Once the one shot signal OS 590 transitions to a logic low value, theinverted one shot signal OS transitions to a logic high value and thecurrent limit threshold signal U_(ILIM) _(_) _(TH) 548 begins todecrease.

Referring now to FIG. 6, a timing diagram 600 of various examplewaveforms of signals of the controller 300 of FIG. 3 is shown. It shouldbe appreciated that similarly named and numbered elements referencedbelow are coupled and function as described above. Further, thewaveforms described in the timing diagram 600 illustrate an example inwhich the current limit threshold signal U_(ILIM) _(_) _(TH) 648 may beclamped within the current limit threshold range 665 to the minimumcurrent limit threshold U_(TH) _(_) _(MIN) 658. The current limitthreshold range 665 is the range of values between the minimum currentlimit threshold U_(TH) _(_) _(MIN) 658 to the maximum current limitthreshold U_(TH) _(_) _(MAX) 656 which the current limit thresholdgenerator 328 may vary the current limit threshold signal U_(ILIM) _(_)_(TH) 648.

As shown in the example, an enable event is received at the beginning ofswitching period T_(N) and the power switch is turned ON. The drivesignal 644 transitions to a logic low value and the power switch isturned OFF when the current sense signal 642 reaches the current limitthreshold signal U_(ILIM) _(_) _(TH) 648. Switching period T_(N) issimilar to the switching periods T₄ through T₈ described above. In theexample shown, the current limit threshold signal U_(ILIM) _(_) _(TH)648 increases, within the current limit threshold range 665, to themaximum current limit threshold U_(TH) _(_) _(MAX) 656 and begins todecrease when the discharge signal DIS 694 transitions to a logic highvalue.

An enable event is received at the beginning of switching period T_(N+1)and the current limit threshold signal U_(ILIM) _(_) _(TH) 648 is stilldecreasing. The drive signal 644 transitions to a logic high value andthe power switch S1 110 is turned ON. When the switch current I_(D) 140represented with current sense signal 642 reaches the current limitthreshold signal U_(ILIM) _(_) _(TH) 648, the drive signal 644transitions to a logic low value and the power switch S1 110 is turnedOFF. As previously discussed, the current limit threshold signalU_(ILIM) _(_) _(TH) 648 begins to increase at the end of the ON time(t_(ON)) of drive signal 644. During switching period T_(N+1), the valueof the current limit threshold signal U_(ILIM) _(_) _(TH) 648 is lessthan the maximum current limit threshold U_(TH) _(_) _(MAX) 656 andcurrent limit threshold signal U_(ILIM) _(_) _(TH) 648 increases for theentirety of the fixed time period of the one shot signal OS. As such,the charge signal CHG 692 is logic high for as long as the one shotsignal OS is logic high.

At the end of the fixed time period, the current limit threshold signalU_(ILIM) _(_) _(TH) 648 is greater than the minimum current limitthreshold U_(TH) _(_) _(MIN) 658 and the inverted one shot signal OS 691transitions to a logic high value. The discharge signal DIS 694 is logichigh and the current limit threshold signal U_(ILIM) _(_) _(TH) 648begins to decrease within the current limit threshold range 665.However, when the current limit threshold signal U_(ILIM) _(_) _(TH) 648reaches the minimum current limit threshold U_(TH) _(_) _(MIN) 658, theoutput of comparator 384 transitions to a logic high value and thedischarge signal DIS 694 transitions to a logic low value and thecurrent limit threshold signal U_(TLIM) _(_) _(TH) 648 remainssubstantially equal to the minimum current limit threshold U_(TH) _(_)_(MIN) 658. The current limit threshold signal U_(TLIM) _(_) _(TH) 648remains clamped to the minimum current limit threshold U_(TH) _(_)_(MIN) 658 until the end of the next ON time of drive signal 644. In theexample shown, the current limit threshold signal U_(TLIM) _(_) _(TH)648 begins increasing at the end of the ON time (t_(ON)) duringswitching period T_(N+2).

FIG. 7 illustrates an example controller 700, which is another exampleof controller 100 shown in FIG. 1 in accordance with the teachings ofthe present invention. It should be appreciated that similarly named andnumbered elements referenced below are coupled and function as describedabove. Controller 700 shares many similarities with the controller 300shown in FIG. 3, however, controller 700 includes transistors coupledtogether as current mirrors that are coupled to charge and dischargeswitches S2 774 and S3 778. In addition, the current limit thresholdgenerator 728 utilizes the current mirrors to clamp the current limitthreshold signal U_(TLIM) _(_) _(TH) 748 to either the minimum currentlimit threshold U_(TH) _(_) _(MIN) 758 or the maximum current limitthreshold U_(TH) _(_) _(MAX) 756 rather than the comparators and ANDgates shown in FIG. 3.

Drive circuit block 726 is similar to drive circuit block 326 shown inFIG. 3. The drive signal 744 is output from the latch 766 to themonostable multivibrator 770. Similar to monostable multivibrator 370described above, the monostable multivibrator 770 generates a pulse witha fixed time period (in other words, the pulse is logic high for a fixedtime period) in response to an edge of the drive signal 744. Forexample, the monostable multivibrator 770 generates a pulse with a fixedtime period in response to the falling edge of the drive signal 744. Theoutput of the monostable multivibrator 770 is referred to as the oneshot signal OS 790.

The output of the monostable multivibrator 770 is further coupled tocontrol switching of the switch S2 774. In the example shown, the signalthat controls the switch S2 774 is referred to as the charge signal CHG792. In the example shown, the charge signal CHG 792 is substantiallythe same as the one shot signal OS 790. Inverter 789 is also coupled toreceive the one shot signal OS 790 from the monostable multivibrator770. The output of the inverter 789 is further coupled to controlswitching of the switch S3 778. In the example shown, the output of theinverter 789 is referred to as the discharge signal DIS 794. In theexample shown, the discharge signal DIS 794 is the inverted one shotsignal OS 790.

One end of capacitor 780 is coupled between switch S2 774 and switch S3778. Further, one end of switch S2 774 is coupled to transistor 797.Transistors 795 and 797 are coupled together as a current mirror. In oneexample, transistors 795 and 797 are p-type metal oxide semiconductortransistors (MOSFETs). As illustrated, transistors 795 and 797 mirrorthe current I_(C) provided by current source 772. In the example shown,the current mirror formed by transistors 795 and 797 are referenced tothe maximum current limit threshold U_(TH) _(_) _(MAX) 756.

Switch S3 778 is further coupled to transistor 799. Transistors 798 and799 are coupled together as a current mirror. In one example,transistors 798 and 799 are n-type MOSFETs. As illustrated, transistors798 and 799 mirror the current I_(DIS) provided by current source 776.In one example, current source 776 may be a controlled current source.As illustrated in FIG. 7, current source 776 may be coupled to receiveselect signal SELECT 796. In one example, select signal 796 may beutilized to vary the magnitude of I_(DIS) of current source 776. In theexample shown, the current mirror formed by transistors 798 and 799 arereferenced to the minimum current limit threshold U_(TH) _(_) _(MIN)758. In the example, the select signal 796 may select the magnitude ofI_(DIS) of current source 776 in response to the input voltage V_(IN) ofthe power converter. For instance, in one example, select signal 796 mayset a first magnitude for I_(DIS) in response to a first input voltagevalue for V_(IN) 102, and select signal 796 may set a second magnitudefor I_(DIS) in response to a second input voltage value for V_(IN) 102.In one example, a lower input voltage V_(IN) may correspond to a largermagnitude for I_(DIS).

In operation, switches S2 774 and S3 778 are opened and closed to chargeor discharge the capacitor 780 in response to current sources 772 or776, respectively. At the falling edge of the drive signal 744, the oneshot signal OS 790 transitions to the logic high value for a fixed timeperiod and the switch S2 774 is closed. In addition, the dischargesignal DIS 794 transitions to a logic low value and opens switch S3 778.As such the capacitor 780 is charged in response to current source 772with current I_(C) and the current limit threshold signal U_(TLIM) _(_)_(TH) 748 increases. When the one shot signal OS 790 transitions to thelogic low value at the end of the fixed time period, the dischargingsignal DIS 794 transitions to a logic high value and closes switch S3778. As such the capacitor 780 is discharged in response to the currentsource 776 with current I_(DIS) and the current limit threshold signalU_(TLIM) _(_) _(TH) 748 decreases.

However, the current sources formed by transistors 795 and 797 andtransistors 798 and 799 are referenced to the maximum current limitthreshold U_(TH) _(_) _(MAX) 756 and the minimum current limit thresholdU_(TH) _(_) _(MIN) 758, respectively. As the voltage across capacitor780 (i.e., the current limit threshold signal U_(TLIM) _(_) _(TH) 748)reaches the maximum current limit threshold U_(TH) _(_) _(MAX) 756, thecurrent mirror formed by transistors 795 and 797 are no longer able tomirror the current I_(C) provided by current source 772 and will provideless current to charge the capacitor 780. Similar can be said for as thevoltage across capacitor 780 (i.e., the current limit threshold signalU_(TLIM) _(_) _(TH) 748) reaches the minimum current limit thresholdU_(TH) _(_) _(MIN) 758. Thus, as the voltage across capacitor 780approaches the maximum current limit threshold U_(TH) _(_) _(MAX) 756 orthe minimum current limit threshold U_(TH) _(_) _(MIN) 758, the rate atwhich the current limit threshold signal U_(TLIM) _(_) _(TH) 748increases or decreases will slow. Or in other words, the magnitude ofboth the increase rate and the decrease rate of the current limitthreshold signal U_(ILIM) _(_) _(TH) 748 will lessen. In one example,the point in which the current mirrors are unable to correctly mirrorcurrent I_(C) and I_(DIS) (the point at which the increase rate and thedecrease rate of the current limit threshold signal U_(TLIM) _(_) _(TH)748 start to reduce), respectively, partially depends on the ratiobetween the channel width and channel length of transistors 795, 797,798, and 799. Eventually, the current limit threshold generator 728clamps the current limit threshold signal U_(ILIM) _(_) _(TH) 748 toeither the minimum current limit threshold U_(TH) _(_) _(MIN) 758 or themaximum current limit threshold U_(TH) _(_) _(MAX) 756.

FIG. 8 is a timing diagram illustrating various example waveforms ofsignals shown in FIG. 7 in accordance with the teachings of the presentinvention. In particular, the timing diagram 800 illustrates an examplein which the current limit threshold signal U_(ILIM) _(_) _(TH) 848 isclamped within the current limit threshold range at the maximum currentlimit threshold U_(TH) _(_) _(MAX) 856 and the minimum current limitthreshold U_(TH) _(_) _(MIN) 858. An enable event is received (as shownby the pulse of enable signal 838) and the drive signal 844 transitionsto a logic high value and the power switch S1 110 is turned ON. When theswitch current I_(D) 140 represented with current sense signal 842reaches the current limit threshold signal U_(ILIM) _(_) _(TH) 848, thedrive signal 844 transitions to a logic low value and the power switchS1 110 is turned OFF. At the end of the ON time, the charge signal CHG892 transitions to a logic high value and the discharge signal DIS 894transitions to a logic low value. The switch S2 774 is closed and thecurrent limit threshold signal U_(TLIM) _(_) _(TH) 848 increases. Asmentioned above, the length of time which the charge signal CHG 892 islogic high may be referred to as the fixed time period.

However, as the current limit threshold signal U_(TLIM) _(_) _(TH) 848approaches the maximum current limit threshold U_(TH) _(_) _(MAX) 756before the end of the fixed time period, the current mirror formed bytransistors 795 and 797 is no longer able to mirror the current I_(C)provided by current source 772 and will provide less current to chargethe capacitor 780. As such the increase rate of the current limitthreshold signal U_(TLIM) _(_) _(TH) 848 decreases as shown by thecurved characteristic of the current limit threshold signal U_(TLIM)_(_) _(TH) 848 closer to the maximum current limit threshold U_(TH) _(_)_(MAX) 856. The shape of the curved characteristic may be partiallydetermined by the ratio between the channel width and channel length oftransistor 795 and 797. Once the current limit threshold signal U_(TLIM)_(_) _(TH) 848 reaches the maximum current limit threshold U_(TH) _(_)_(MAX) 856, the current mirror formed by transistors 795 and 797 providesubstantially no current and the current limit threshold signal U_(TLIM)_(_) _(TH) 848 is substantially clamped at the maximum current limitthreshold U_(TH) _(_) _(MAX) 856.

At the end of the fixed on time, the discharge signal DIS 894transitions to a logic high value and switch S3 778 is closed and thecurrent limit threshold signal U_(TLIM) _(_) _(TH) 848 begins todecrease within the current limit threshold range 865. However, as thecurrent limit threshold signal U_(TLIM) _(_) _(TH) 848 approaches theminimum current limit threshold U_(TH) _(_) _(MIN) 858, the currentmirror formed by transistors 798 and 799 is no longer able to mirror thecurrent I_(DIS) provided by current source 776 and will provide lesscurrent to discharge the capacitor 780. As such, the magnitude of thedecrease rate of the current limit threshold signal U_(TLIM) _(_) _(TH)848 decreases as shown by the curved characteristic of the current limitthreshold signal U_(ILIM) _(_) _(TH) 848 closer to the minimum currentlimit threshold U_(TH) _(_) _(MIN) 858. The shape of the curvedcharacteristic may be partially determined by the ratio between thechannel width and channel length of transistor 798 and 798. Once thecurrent limit threshold signal U_(TLIM) _(_) _(TH) 848 reaches theminimum current limit threshold U_(TH) _(_) _(MIN) 858, the currentmirror formed by transistors 798 and 799 provide substantially nocurrent and the current limit threshold signal U_(ILIM) _(_) _(TH) 848is substantially clamped at the minimum current limit threshold U_(TH)_(_) _(MIN) 858.

The above description of illustrated examples of the present invention,including what is described in the Abstract, are not intended to beexhaustive or to be limitation to the precise forms disclosed. Whilespecific embodiments of, and examples for, the invention are describedherein for illustrative purposes, various equivalent modifications arepossible without departing from the broader spirit and scope of thepresent invention. Indeed, it is appreciated that the specific examplevoltages, currents, frequencies, power range values, times, etc., areprovided for explanation purposes and that other values may also beemployed in other embodiments and examples in accordance with theteachings of the present invention.

What is claimed is:
 1. A controller to regulate a power converter,comprising: a terminal coupled to receive an enable signal includingenable events representative of an output of the power converter; adrive circuit coupled to generate a drive signal to control switching ofa power switch to control a transfer of energy from an input of thepower converter to the output of the power converter, wherein the drivecircuit is coupled to turn on the power switch when an enable event isreceived in the enable signal; and a current limit threshold generatorcoupled to the drive circuit to generate a current limit thresholdsignal, wherein the current limit threshold signal varies each switchingcycle of the power switch, wherein a time between consecutive enableevents in the enable signal is the switching cycle of the power switch.2. The controller of claim 1, wherein the current limit threshold signalincreases in response to the power switch turning off and thendecreases.
 3. The controller of claim 2, wherein the current limitthreshold decreases until a current sense signal representative of acurrent through the power switch reaches the current limit threshold. 4.The controller of claim 2, wherein the current limit threshold signaldecreases until the current limit threshold signal reaches a minimumcurrent limit threshold.
 5. The controller of claim 2, wherein thecurrent limit threshold decreases with a series of decreasing steps. 6.The controller of claim 2, wherein the controller selects a decreaserate for the current limit threshold signal in response to an inputvoltage of the power converter.
 7. The controller of claim 2, whereinthe current limit threshold is coupled to increase by up to a fixedamount when the power switch is turned off.
 8. The controller of claim2, wherein an increase rate of the current limit threshold slows as thecurrent limit threshold approaches a maximum current limit threshold. 9.The controller of claim 2, wherein a decrease rate of the current limitthreshold slows as the current limit threshold approaches the minimumcurrent limit threshold.
 10. The controller of claim 1, wherein thecurrent limit threshold generator includes: a capacitor coupled togenerate the current limit threshold signal; a monostable multivibratorcoupled to generate a one shot signal in response to the drive signal; afirst current source coupled to charge the capacitor in response to theone shot signal, wherein an amount at which a voltage across thecapacitor increases is proportional to a magnitude of current providedby the first current source and an amount of time the one shot signal islogic high; and a second current source coupled to discharge thecapacitor in response to the one shot signal, wherein an amount at whichthe voltage across the capacitor decreases is proportional to amagnitude of current provided by the second current source and acapacitance of the capacitor.
 11. The controller of claim 10, whereinthe second current source is coupled to receive a select signal toselect from a plurality of magnitudes for the magnitude of currentprovided by the second current source in response to an input voltage ofthe power converter.
 12. A controller to regulate a power converter,comprising: a terminal coupled to receive an enable signal, wherein theenable signal includes one or more transitions between a first state anda second state that are representative of an output of the powerconverter; and a drive signal generator coupled to generate a drivesignal to control switching of a power switch of the power converter tocontrol a transfer of energy from an input of the power converter to theoutput of the power converter, wherein transitions of the drive signalfrom a first state to a second state are coupled to turn on the powerswitch, wherein transitions of the drive signal from the second state tothe first state are coupled to turn off the power switch, wherein thedrive signal generator is coupled to control the drive signal such thateach transition of the drive signal from the first state to the secondstate is substantially coincident with each transition of the enablesignal from the first state to the second state, wherein a beginning ofeach switching period of the power switch is substantially coincidentwith each transition of the enable signal from the first state to thesecond state.
 13. The controller of claim 12, wherein the drive signalgenerator is further coupled to control the drive signal such that eachtransition of the drive signal from the second state to the first stateis substantially coincident with a power switch current indicated by acurrent sense signal reaching a current limit threshold signal.
 14. Thecontroller of claim 13, wherein the drive signal generator comprises alatch coupled to generate the drive signal, wherein the latch is coupledto be set in response to each transition of the enable signal from thefirst state to the second state, and wherein the latch is coupled to bereset in response to a comparison of the current sense signal and thecurrent limit threshold signal.
 15. The controller of claim 13, furthercomprising a current limit threshold generator coupled to receive thedrive signal to generate the current limit threshold signal in responseto the drive signal.
 16. The controller of claim 15, wherein the currentlimit threshold generator is coupled to increase and then decrease thecurrent limit threshold signal in response to each transition of thedrive signal from the second state to the first state.
 17. Thecontroller of claim 16, wherein the current limit threshold generator iscoupled to increase and then decrease the current limit threshold signalwithin a current threshold range.
 18. The controller of claim 16,wherein the current limit threshold generator is coupled to decrease thecurrent limit threshold signal until current limit threshold signalreaches a minimum current limit threshold.
 19. The controller of claim16, wherein the current limit threshold generator is coupled to decreasethe current limit threshold signal with a series of decreasing steps.20. The controller of claim 16, wherein the current limit thresholdgenerator is coupled to select a decrease rate for the current limitthreshold signal in response to an input voltage of the power converter.21. The controller of claim 16, wherein the current limit thresholdgenerator is coupled to increase the current limit threshold signal byup to a fixed amount when the power switch is turned off.
 22. Thecontroller of claim 16, wherein the current limit threshold generator iscoupled to slow an increase rate of the current limit threshold as thecurrent limit threshold approaches a maximum current limit threshold.23. The controller of claim 16, wherein the current limit thresholdgenerator is coupled to slow a decrease rate of the current limitthreshold signal as the current limit threshold signal approaches aminimum current limit threshold.
 24. The controller of claim 15, whereinthe current limit threshold generator comprises: a capacitor coupled togenerate the current limit threshold signal; a monostable multivibratorcoupled to generate a one shot signal in response to the drive signal; afirst current source coupled to charge the capacitor in response to theone shot signal, wherein an amount at which a voltage across thecapacitor increases is proportional to a magnitude of current providedby the first current source and an amount of time the one shot signal isthe second state; and a second current source coupled to discharge thecapacitor in response to the one shot signal, wherein an amount at whichthe voltage across the capacitor decreases is proportional to amagnitude of current provided by the second current source and acapacitance of the capacitor.
 25. The controller of claim 24, whereinthe second current source is coupled to receive a select signal toselect from a plurality of magnitudes for the magnitude of currentprovided by the second current source in response to an input voltage ofthe power converter.